A fan apparatus positioned upstream of radiator has been used. FIGS. 15 and 16 show the conventional fan apparatus 100 including a fan blade 101 and a fan shroud 102. Comparing with the fan apparatus 200 positioned downstream of the radiator 5 (shown in FIG. 17), the fan apparatus 100 positioned upstream of the radiator causes more noise. FIG. 18 shows the test result comparing the fan apparatus 100 with the fan apparatus 200. The solid line Q of FIG. 18 indicates the fan apparatus 100 positioned upstream of the radiator, and the dot line R of FIG. 18 indicates the fan apparatus 200 positioned downstream of the radiator.
It is presumed that the air generated by the fan apparatus 100 cause more turbulent flow than that of the fan apparatus 200. In order to reduce the turbulent flow of the fan apparatus 100, a suction ring 112 has been proposed to be positioned in front of the fan blade 101, such as shown in FIGS. 19 and 20.
However, it is very hard to determine the best design of the shape of the front edge of the suction ring 112 and the position of the suction ring 112, because of the turbulent flow is unsteady in accordance with the flow caused by the fan blade 101. Furthermore, since the suction ring is positioned within the air flow as, the suction ring itself causes the resistance of the air flow. Accordingly, the suction ring 112 reduces to the blowing efficiency. Furthermore, the suction ring 112 itself makes a noise.